1. Field of the Invention
The present invention relates to a plastic article, a method of shaping the plastic article, and an optical scanning device including the plastic article, and more particularly, to a plastic article used for an optical scanning system of laser type digital copier, a laser printer, or a facsimile machine, an optical device of a video camera, and an optical disk.
2. Description of the Background Art
Plastic articles can be formed using various shaping methods. For example, an injection molding method can be used to form plastic articles with given shapes. Specifically, melted resin material such as hot-melt resin is injected to fill a cavity, having a given volume, defined in a metal die, while the temperature of metal die is set around a given temperature at which the resin can be deformed by heat. Then, the metal die is gradually cooled while controlling the internal pressure of metal die. After the completion of cooling, the metal die is opened to extract out the molded product.
Such injection molding method can be cost-effective for mass production of a plastic article having a special shape, using the metal die adapted for such special shape of the plastic article.
Optical elements, such as lenses and prisms, have heretofore been mainly made of glass, because high degree of precision is demanded for the optical face and internal birefringence. With demand for further cost reduction of products, materials used for forming optical elements have been shifted to plastic, and plastic lenses, plastic mirrors, or the like have come to be widely used.
The plastic article may be shaped into various shapes in view of the application fields of the plastic article. For example, a plastic article having a portion of reduced thickness as a precison transfer area can be formed by transferring a minute and complex concave/convex shape to the transfer area, in which high degree of precision transfer of the metal die shape to the transfer area of the plastic article is required. For example, in the case of the lenses (e.g., f-theta lens) used for an optical scanning system of a laser printer, the shapes of the lenses are designed into a spherical shape or a complex non-spherical shape to reduce the number of parts while maintaining a plurality of capabilities with a limited number of parts. Further, such lenses may be made thin for greater compactness of the apparatus.
The process of shaping the plastic article includes cooling and solidifying the melted resin in the cavity of metal die, and the plastic article can be formed into a desired shape with high degree of precision by maintaining a constant, uniform pressure and temperature within the cavity. However, the speed of cooling and solidifying of the plastic article having a complex uneven thickness shape differs among different portions of the same plastic article, and internal stress may cause sticking of the article to the metal die or other separation failure when the plastic article is removed from the metal die, and the shape deformation such as warping of article after the removing process may occur. For example, internal strain of plastic optical elements may cause birefringence of plastic optical elements.
For example, a plastic article 10 shown in FIG. 1 may be formed or shaped as follows. The plastic article 10 has a transfer face 11, and a cross-sectional face 14 perpendicular to the transfer face 11. The plastic article 10 has a first thickness “a” in a direction perpendicular to the transfer face 11 and a second thickness “b” in a direction parallel to the transfer face 11.
Even at the portion of reduced thickness of the plastic article 10 having the aspect ratio of a/b<1, because the cooling speed in the first thickness “a” direction is faster than the cooling speed in the second thickness “b” direction, the plastic article 10 may be cooled and solidified while retaining the pressure on the transfer face 11, internal stress may cause sticking to the metal die or separation failure when the plastic article is removed from the metal die, and the shape deformation such as warping of article after removing process may occur.
Hereinafter, when plastic articles have a relation of “a/b<1” for the first thickness “a” in a direction perpendicular to the transfer face 11 and the second thickness “b” in a direction parallel to the transfer face 11, such plastic articles may be referred to as a plastic article having thin-thickness shape such as having a portion of reduced thickness or thin-thickness shape plastic article.
Low-pressure injection molding using the lowest possible pressure (called low pressure injection molding) needs to be conducted to reduce the remaining pressure. However, in low pressure injection molding, because the amount of resin injected into the mold is small compared to the cavity volume, shrinkage may likely occur to a plastic article formed by such molding, and due to an increase of resin volume shrinking, the precision with which the shape of the metal die is transferred to the plastic article (hereinafter “transfer precision of the metal die shape”) deteriorates.
In view of such shrinkage, JP-H06-304973-A discloses a method of using a gas port, in which air having a given pressure is applied to a non-transfer portion of an article through the gas port to set a pressure difference between the non-transfer portion and a transfer portion of the article to induce the shrinkage at the non-transfer portion of article, by which shrinkage occurrence at the transfer portion may be prevented.
Similarly, JP-H11-28745-A discloses a method of moving one block among the blocks composing a metal die, in which a cavity is defined by the blocks. When an article is formed using the injection molding, resin is injected into the cavity to fill the cavity and a certain pressure is created in the cavity, and a transfer face of the article resin is maintained in close contact with the block at a suitable pressure. While forming the article, one of the blocks defining the cavity is slidably moved in one direction to separate a non-transfer face of article resin from the block. With such block movement, a space is set between the block and the non-transfer face of article resin, by which the shrinkage can be induced at the non-transfer face of article resin.
Further, JP-2000-84945-A discloses a method of forming a plastic article having a thick thickness and/or uneven thickness shape, in which an incomplete transfer portion of an article is set at one portion of the article other than a transfer face of the article, and a concave shape or a convex shape is transferred at such incomplete transfer portion of the article by transferring a cavity shape defined by the metal die, by which the remaining resin internal pressure and internal strain can be reduced.
The shrinkage may occur when the remaining resin internal pressure after the injection filling and resin cooling becomes lower than the air pressure applied to the non-transfer face or portion.
When a plastic article having thin-thickness shape such as having a portion of reduced thickness is formed by the method of JP-H06-304973-A, the resin at the thin-thickness portion is cooled and solidified under high pressure at the earlier stage of cooling and solidifying, by which the internal stress and internal strain remains in the article, and thereby the transfer face precision deteriorates and birefringence deteriorates.
Further, when a plastic article having thin-thickness shape such as having a portion of reduced thickness is formed by the method of JP-H11-28745-A, a separation process of block from the resin is required before a transfer face portion of resin is cooled and solidified, by which the cavity volume is increased as the separation process proceeds and the pressure in the cavity becomes a negative pressure. When the negative pressure occurs to the resin, an adhesiveness of transfer face of article resin becomes low, by which shrinkage may occur to the transfer face of article resin. In light of such situation, a movable block may be used that can follow a volume shrinking of resin. But it is hard to move the movable block precisely while maintaining the stability of the movable block.
Further, when a plastic article having thin-thickness shape such as having a portion of reduced thickness is formed by the method of JP-2000-84945-A, similar to JP-H11-28745-A, when a separation process is conducted before the resin is cooled and solidified, the negative pressure occurs to the resin in the cavity, by which shrinkage may occur to the transfer face of the article resin. Further, even if the shrinkage is induced after the pressure becomes a low level, the cooling and solidifying of resin has already proceeded while the internal stress remains.
Further, the method of JP-H06-304973-A may not completely control a shrinkage inducing area, by which the shrinkage may spread to the transfer face of the article resin. Further, the methods of JP-H11-28745-A and JP-2000-84945-A may not completely control a shrinkage area within the separated block area, by which the shrinkage may spread to the transfer face of the article resin.
The progression or spread of shrinkage area to the transfer face is described with reference a plastic article 10 shown in FIG. 1 to FIG. 3. The plastic article 10 having a transfer face 11 is an article formed by injection molding.
FIG. 2 shows a cross-sectional view of the metal die 30 used for forming the plastic article 10 shown in FIG. 1. FIG. 2 shows a cross-sectional view of the metal die 30 cut at the cross-sectional face 14 of the plastic article 10 made of melted resin such as hot-melt resin 37. The metal die 30 may include at least a pair of metal dies, which can be opened and closed in a given direction, and a cavity is defined by the metal dies. The plastic article 10 can be formed by injecting the hot-melt resin 37 into the cavity. For example, the cavity is defined by an upper transfer block 31, a lower transfer block 32, and a side block 33. The side block 33 may be disposed with a gas slit 35 and a gas port 36 communicated to the gas slit 35. The gas port 36 is connected to a gas compression unit disposed outside of the metal die 30, by which the compressed gas (e.g., air) can be guided to a side face of the cavity. In the method of JP-H06-304973-A, the metal die 30 is used for forming an article by applying an air pressure to the non-transfer portion through the gas port 36, in which the shrinkage is induced to a non-transfer face 22 of the article. However, the shrinkage area may not be confined within the separated block area such as block 33, and the shrinkage may spread to the transfer face 11 of the article resin (see a circle P of FIG. 2).
Further, FIG. 3 shows a cross-sectional view of the metal die 30 for forming the plastic article 10 shown in FIG. 1. FIG. 3 shows a cross-sectional view of the metal die 30 cut at the cross-sectional face 14 of the plastic article 10 made of the hot-melt resin 37. The metal die 30 of FIG. 3 includes a movable block 34 instead of using the gas slit 35 and the gas port 36. Such metal die 30 can be used for the method of JP-H11-28745-A, in which the movable block 34 is slidably moved to induce the shrinkage to the non-transfer portion of the article resin. However, as similar to a case shown in FIG. 2, the shrinkage area may not be confined within the separated block area such as block 33, and the shrinkage may spread to the transfer face 11 of the article resin (see a circle Q of FIG. 3).